Vapor generator



Nov. 16, 1943.

M. l--RlsczHv VAPOR GENERATOR Filed May. s, 1939 2 sheets-sheet 1 BWM ATTORN EY Nov. 16, 1943. M. FRlscH VAPOR GENERATOR Filed May 3, 1939 2 Sheets-Sheet 2 FULL LINES FIRING yFCIF CONSTANT SUPERHEAT' TEMPERATURE DOTTED LINES EQUAL FIRING OF BOTH FURNACES ooo STEAM 72' R loo |20 Y ,INVENTOR /VART//v /e/ che EY ATTORNEY Patented Nov. 16, 1943 `airline 2,ss4,1'1 varon GENERATOR Martn Frisch York, N; Y.; assignor to-Fos terWheeler Corporation, New York, N. Y., arcorl VporationoffNewrork n applieancnmay-3, 1939, seriaiNamLesz;

iijlolaims, (C1. 12a-235) Y This inventionfrelates to@ vapor generators, and more `vparticillarlv; pertansltogapparatus for theA control` or superneatf; overf ai Wide ranger of loadsin steam: generators; utilising a.- plurality of separate'urnaces operatingrinzporallel.

Tneinvention isbasediuoon the dlscoverythai when, a steam :boiler isgredf'fby'twoor moreffurf heating meansy camrrioiilvfY naces; operating: inyparallel; andseacha furnace yis provlcledi witn heat; absorbinevmeans suoli, as: water walls or-xothen wall; cooling. surfaces, vL and the lool-1erv heating surfac e isi so'farranged; with respectto :the furnaces thatcthegasesleavng the furnaces mix; even; lmperfeotly, before entering the boiler;A heating; surfaces the: temperature of the mixedY gases is lowestzwhen thmfurnacesiiare.

evenly` red, and,` increases Whenthelurrlaosrare nnevenly.V rech For. example; :if an steam boiler fireclrby tv/o furnaoesf,', s.op eratedf att half load, and fl this-loadis: developed; by-ffiri-ngcf one.:y half of` the required ,fuel .ina each ofi the;V4 furnaces; the temperature of` thegajses entengme boiler; :will

` be consideralzly,A lower: thani if f the; loadofls;devel:-

employed ine Sbelm boilersvrecl asinglegfllmaoel, Y A

one such` meansf. heretofore, used is; amsnnerev heater having.; 'both conveotionand radio; theat absorbing sections connected, imseresi or: minare.

enel.-A Anotnerf suoni means includes an .overf- Size -conveeionfsllperheater' which aiiorrnallsfi elves lull'y load.` an'd.- orf wlelos=4 the` desiredl. steam perature at somo predetermined; lower; loa@ bypeesrinrn-ara1iei 'vv-ith,.- tne; supernews, is, ne', lizedso metal: loadsebove thelow'loodffcWlfioh the sunerheater isv desi ne@ a -eertaaarnnritnf to keeprthe-sunerheat dowr1r-to-thea-oe`sired level.- wheni such an arrangementen emoloyeowitlr single -fnrnace boilers, tbesugerheatenisneoese sarily: large in order 7to .Drolllllbilgw: desiroldrstem temperature at the; predetermined l low loafcl;V beecause oiitnerapili ty -vvitt1 A thigh-,the temperature of `one gaseslenteringrtnasuperheater;decreases 'of` gases that mustl be byspasseclv `at with, decreasing-loads. Goneollentlytheyolume ighh loadss Genuine;

the predetermined: clesirerlj,level,V is

However, when the present, invention" isl nti-V f lizecl end ,twc or more serarate iurn'aoe'sae fired enteringftne boiler will cbehig-her. When, the

is desiredto maintain vthe maximum {p ossble; gas4 temperature through thelboilerorieffurnace is redat its fnllloagl, Avvhich-Ucorresponds tof'one half of full boiler load, Whilethe: other f urnaee is red, to;` makeup, the y difference With this method of opereton, the, temperature of the gasesenterlng thenboiler, from %l to 100% of loaidfwill. always behigher-thanfibwould be if; the equivalent .loa-,d is` developed byr firing: an. equal amountoiluel lnreach of thetvvo Vfurnaces.

In accordanceivrithgthe presenti inventionthe eloplcation` of this 4principle -makes iti-possible to obtain; acomparatvdyyat temperature curve for the gases entering: the sunerheaersection of tbe'boiler, andhence tofrealize aisubstantially y constant. nal; steam." temperature over a wide range. of i 1oads.htl1erto;- obtainableY,` ronly by; theA usefon more: eiznensiveand cumbersome superonrerennauy, nie desiree. eteamtemnerarnre may loeV obtainedA at', af predetermined low load with considerably I esssuperheatrlg., Slfaei dll@ to tnelactthat-tbegasiternncrature entering the superbeaterE changes .vevlittle .from,ha1 tafllll load.. Therefore, inordertoicortlol @Clirateli the! temperature of.` tnosteami above. half: load; apprecably lesSby-l-paissrlg goed; done ill. bQl

ers -fred ,by two Yor-` more fnrnaces. an 1ers firedby a s inglefj nace; asarresilltofwhioh tnesunerheater. end byl-oass-lmeans; earllbe sub.- etanolally smaller; and lesseXDenslveQ.

i mneeharacrensncs. of;the.,interinon;vvillfilelnnT derstooclf. from the. following descriptionwhen consiclered..inl-cormectiomwiththe aceorrjnanylns Fig.` 111e,k a. vertical sectional view more or` less diagl?alrlrllatically.y a, erator embodying one.. forIIALV Qfltll and j Fig-. 2 is; ai chart showing, a1gronnrof-tcurves typical ofi the varia lons,in easl-anc1'steaxn temperature obtained, overa wle frange ofloads, and! illustrating, graphicallytlie. eleet' on; these ternp eratiires` ol differentially controlled? and eoilal rine oftwofurnacesina steaml'generotor embodying', the ,form ofi the.. irxlventlonhovm in Figi y L 1 f considerable Likecharacters of rreference refer to the same or to similar parts throughout the several views.

Referring to thefdrawings, particularly to Fig. l, reference character |63 designates generally the settingl of a steam generator having two 5 are connected at their upper ends to a boiler y drum 2|, and at their lower ends to headers 22 and 23 respectively. The rows of tubes is andl extend between and normal to the front and rear furnace walls and thus provide a partition which divides the setting into two separate furnaces which are arranged for parallel operation. Water wall tubes 24 and 25 are disposed on the .7 inner 'surface of the side walls l5 and I6 respectively, witl1-thetubes24 being connected at their upper ends to a header 26'and'at their lower ends tofheader `21,-and thetub`es`j` being similarly connected to upper andlowerfheaders 28 and 28. The lower headers 22, 23; 21 and 29 are connected into the `boilencirculatin by suitable connections', not shown. Water wall tubes 36 and 3i line l the inner surface of the furnace roofl portions 32' and '33 respectively, andare connected at'their vlower ends to the headers 2.6- and 28 and at their upperendsto headers`34 and 35 respectively. -jA centrally located steam and water drum 36 is connected with a steam and water drum 31jat oneH sideof the setting by circ'ulators 38 and 39, and connected with another similarly disposed steam and water drum 48 on the `other side ofthe setting, by circulators 4| and 42. `A vertical rowof downcomer tubes 43 connects the druml 31 withfvertical headers 45, and a similar row'offdwncomer tubes 44 connects the drum 4f! with' verticalheadersAG. Groups of steam generating tubes 41and 48, spaced relatively widely apartgconnect the headers 4 5 and 46 respectively, with fthelfcentral s tea'nfi` and water drum` 36. Groups 'of inclined screentubes 49 and 56 extend across the jpath of the .gases leaving the-furnaces; between'ithejdrum 2| andthe headers 34v and 35 respectively, :and have theirvupp'er ends connected to drumj 31.` The innerrsurfaces o-f the rear wallsr I1 'and I8 ofthe furnaces are lined' respectively by groups of water wall tubes 5| and 52,"` 'I The""tubes`5| .are connected to an upper `li.'1'rizor1ta1 header 53 and a lower horizontal header 54; while the tubes 52 yare connected t0 5,5 an, upper horizontal header 5 5 and a lower horizontal header 5'6.

I A group lof short water wall tubes 51"`also line portions of the inner surfaces of the rear furnace walls, and these tubes are connectedt'oan Vupper header 56 and to allower 6 I ,header 59. It will be understood that the fronty furnace' walls, not shown, may also be kprovided with. similarigroups of water walltubes. .fOn Opposite 'sides of the, upperportion of the setting and beneath the circulator tubes 39 and 42 are economizer sections 68 and 6|. Feed waterfrom any suitable source enters the inlet headers'BZf'and 63 andows irrparallel through each of ..th,e V.economizer lsections `to outlet headers f64.and 65 respectively, andthence by conduit `connections,v not shown, to the boiler circulation. Header 34 is connected to the steam and water drum by arow of `spaced boiler tubes 66, ythe lower portions of'which are horizontally inclined, and the header 13,5 is connected tothe l) and I6 and parallel thereto are two rows of closen l ly spaced vertical water tubes i9 and 26, which drum 36 by a similar row of spaced boiler tubes 61. In the space between economizer section 66 and the lower horizontally inclined portions of the tubes 66 is disposed a superheater section 68, and on the opposite side of the setting is a similarly disposed superheater section 69. Saturated steam from the drum 31 flows through steamvline 16 to4 superheater inlet header 1| of superheater section 68 and thence through that section to the outlet header 12. At the same time, steam line 13 conducts steam from the drum 40 to inlet header 14 of superheater section '69, and afterV flowing therethrough, the steam flows into outlet header 15. From the superheater outlet headers'12 Vand 15, the superheated steam is conductedpto its point of use by suitable means, .not shown. v

A by-pass for the gases around the two superheater sections y68 and 69 is provided in the zone through which pass the vertically disposed portions" of the tubes 41, 48,` 66-and 61.v A'S shown,

ly' away from the tubes 41 and 48, where these baffles bend inwardly to the-outermost tubes 41 and 43. and continue upwardly therealong to the drum-36. One or more openings are provided in eachbalile V8|) and 8|` adjacentY their upper ends, which are controlled by dampers 1.6 and 11 respectively, toregulate' the quantity of gases flowing through the by-pass and around the super- Y l heater section in-one or both of the gas passes. Baffles :1-8 and 19 are disposed along, or adjacent to, theuppen ends ofr tubes llv and 6.1, and .extend-from the drum 36 downwardly'. nearly to ythe bottom 'of economizer-s 60 and 6|,V whereby gas -passagesare provided through which gases from the by-'pass enter the gas passes in the zone of the economizers l and 6|. ISuitable means are provided to operate thedar'npers 16 and 11, either individually or in groups, to providethe desired degree of regulation-of the flow of gases i through the by-pass and 'over each or both secover the economizer.sectionsand thereafter leave the' boiler through the ues 82 and 83.

4As shown diagrammatically in the drawings, fuel in the form of pulverizedcoal is fed tothe burners |3Hand |4 through fuel lines 84 and 85 from pulverizingmill 86. Variable amounts of fuel may'A be supplied toeach burner, and the relative rates ofy firing of the two furnaces thereby .controlled by means 4of a fuel proportioning valvex'directly connected toA the ldischarge of mill exhauster 88,v or, if desiredftwo separate .controlling exhaustersmay be-usedf-with one" mill, or 4separate mills for each furnace may be employed.` Obviously, the furnaces may be fired by other fuels, ifl desired. y In operation, vthe gases leaving 'thertwo' furnaces will mix in'the space kA before enter'ingthe boiler proper. ABefore entering the 4superheater sections',- the mixed gases pass over the widely spaced boiler tubes 41 and 48,-a's'a result of which the temperature of the gases isl reduced toiapredetermined valueat maximum vload above which the temperature shouldV not rise, in order to minimize slagging. At loads below maximum, the temperature'of the gases entering the superheater sections, and the final lsteam temperature, lis maintained as nearly constant as possible by differentially firing the two furnaces. superheat control'is obtainediby causing desired Additional eteamz temperatures. for aifferemiany continued andgequal ringof thertwo furnaces; `as'wivellfas the effect of. the-by-.pass'f onztlie '.iinal'. steam tem:k perature;` vThemlotted lineY.curveslshowf-thegeiect on'.performancefatzalliloadsotfequally.iiringeboth furnaces, `under..Whiclaoconditions;.tide lnatural l characteristics; offfthegsuperiieater determine the final steam temperatures. show; the .eiiect on periorrnfancef:whenfiring l the furnaces. differentially; to.- obltain: thea degreei of superheat indicated` byf tlfielitviucurves. De l The uppen curve-D 'showsrtlae nal' steam-:temperature obtained by diiierential firing aloneethat iswifthi out the use/cf the.by-passi` While the @lower-:curve D. shows triennal steam temperatures Whichmay befobtainedby. 'differentialijringi andthe use.'` of the by-passin .the .range between` 50% and2120.% of.Y full load. Curve E; showsthe. percent; 0i` total gas! byepassedat.^ the various@y loads Within.. thisrange. The curves-MAi show.thatemperaturesp tiievinixed furnace.- gaseszbefore entering-fthe.k boiler, and indicate. the-difference in; tneseitemperatures when the furnaces-s are, iired'-` equally, ancla when they are fired:diiierentially..` l Curves Bfshow the temperature of the mixedi` gases.` entering the The full line curves` heater. VBy dilerentiallyz.l firing.: the furnaces Y as hereinbefore:.describediand bye controlling the ow oil :gases:` through .the by-pass, a constant.; or sub-4 stantiallyconstantfidegree oisup'erheatimay be obtained overawideload rangei; M

. Theypartieudiar: l`arrangeiauenti andi disposition of the steam i generating; andi superhe'atingl sections ofetlaeffformsl .or the :inalexititonlselected` for; illus;- trationV may beevariedimithout .departing iromzthe pi#incipit-1s.:of"I the invention. Eori example;Y if:V desired; more than-ltwo;furnaces.m-ayaibe,;.employed.

Meansrother, tha-ne that'di-sclesed may berV utilized forcontrolling.thezrate oir. ring of ythe -burners in: the" two` furllaces.;v Such means 4may.-include automatic means responsive to the steam temperatureitol; provide unequaliiringfinthe desired dileetien; and. .automatic m eansresponsiveto .the

' pressurefso; asatoA maintain ,thegii-ring. ratei'at `the djesiredgvalue,toiproducefthefdesiredisteam 110W.

Moreffthanone@ r'burner-A may,` of. course,y be futilized Since; the;.linventi onis not limited to thefforms selected forillustration,v it lwillbeunderstoodfthat the invention r is not to f be limited excepting by the` scope; offthe:appendedgclaims..

1. Vapor-vl generating,- apparatuscomprisi-ng; a setting, apluralityy of Lfurnaces-.ini the setting arzranged;` to l operate in.; paral1el, meansi for-:g iiring each furnace-Z at Various.iint,f-:nsifties,l awaterrwall Y in., each furnace', -a L mixing; chamber into .which the EQSQS .from each furnace fiovv,` andi mix-With the: gases-from `another` furnace, vmeans-forming af. gas passage throughI which the lrrlixedf` gases flow from said mixing chamber, vaper'generatingsu-rface inthepath vof the mixed ygasesflovvsuperheater sections.; andithe.;di-flerencein the temperature of Athese` gases `at.l variousiloads-Lefffected by equal or: by diiierential ming..r vTlie curve C sliows` the.gas; temperature; leaving the superheater sections.; :j .l

Trie` curves `of :FigfZ'shoW clearly.v that .wliemthe Y twoiurnacessare firediequally, the.; temperature of the gasessleavingethe: furnaces andzthe teme. perature. oi the` gases entering the f superheater are materially` lovveifthan.corresponding.` temper.- aturesY of4 thea gases` Whentheffurnaces arefrred dierentialiyu Diiere'ntial-Y ring accordingwto Fig. 2,; includes .the firing of :but` one furnace; for ioadsup to halfiload; and.. for loads between.r half andlfullfload, the'ringof oneurnace at anim. tensitysto produce half?load.` Witlt` the -ring., of the other iurnaceY at 'allaintensity-reoluiredy for the-.desired load. Wildenthgby.-pass4 isinot L used, the iinalfsteam temperatura` orwthez degreeof superheati irnpartedto` the steamfwilflfvary but slightly` between 50% andi .1-00i%1of-2 ullload,l as shown-by the uppercurvefD;L leyendlf` full load tnelftemperaturerises. However,`V byusingY the ley-pass, the n'al steam temperature at a-llloads above half load rmay .l be maintained constant. as indicated-by the-lowerL curve D; Consequently, With-l diierential ring ofi theiurnaces, a-substantially constant degree oie superheatv may. be obtained atvaricus loads, andi-by diierential'iiring with the-use offlthewbyepass; constan-t superheat mayfb'emaintainedatrvarious:loadsl` l 'f inglfrom. Vsa-idcharrrbe'rl and; connected-f to said Water` Wallslin saidiiurfnaces, mean-sj forming. a

bYl-passzin the central portion; of.. sadgas.passage;L means ier" controlling.- the',l iiowl oil gases through the byfpass,r and-superheating'surface in said gas passageon vlcothsidesof4 saidy byfpass overb the 'gases from. saidemixing f chamber flow.: inl.- passing; through said. passage, tlie arrangement beingA such tha-tl the fsuperlieatg` tem;- perature is controlled by'unequally `firing the'giurnacesfto` regulate the tern-nerature-.of tliegas mixture rand* .by regulatingv the.: quantity of gases flowing through Asaid .by-passi 2. Vapor1 generating apparatusA comprisingwa setting; aipluralityof furnacesin vthefsetting.Sar.- g ranged. temperate,- inA parallel, means: for] riug Y each.. furnace at VVarious intensities amatenwall =in .eaclrfurnace a chamber intozwhich .thegasesmm each furnace'liowrandmix with the gases fromanothenfurnace', means forming a. gas. .passage.-through:=wl'iich the` mixed gases flow, i-romrsaid mixingI chamber, .vapor generating surface in: the path..of';the miXedmga-sesfliowing From-the ioregoing it will :be perceivedathat v inaecordance with theiinVen-tion superheat I-control may :bei obtained readily irr-si'ieani` generators red bya-pluralityfof--Water cooledeiurnaces so arrangedl that thehgases flrom-'tlrerfurnaces lmix l priortoi'enteringl-5tlie steam f generatinggsurface and- .priori to.'y coming into-contact1with thesuper from; saidfcham'ber. and? `connectedite. said :water Wallsinsa-id furnaces, meansfformingna lay-pass inf theFcentra-lportion .of said igasipassagameans Vforcontrolling.the flow oi gases through thebypass.;superheatinggsurface zin said gas. passage on both` ,sidesgflofg said-.flcys-passf over @whichftlie gases from said; mixing chamberf flow in passing through `said passage, and heat-*recovery .surface in saidffpassage. in :therpathr of the gases 'flowing from the* superheating: surface 'andj from said byepass.. l

3:. Vapor i generating apparatus-comprising a setting, a plurality of Afurnaces in--tlres''tting ar.- rangedto:V operatewin parallel;- meansufor'iiring each furnacefat -variouslinterisitia a "Water wall in.; eachi u furnaces-v` a mixing chamberfintd ywhich -unequally firing the furnaces/to regulate the temperature of the gas mixture and by regulating the quantityof gases flowing` through 'said bypass.' l Vapor' 'generating apparatus comprising a setting, a plurality of furnaces in the-setting arranged` lto operate in parallel, means for-ring each furnace at various 'intensities, a' water,` wall ineachfurnace, a 4mixing chamber into `which the gases from each furnace flow'and mix with the gases fromanother furnace, means forming a gaspassagethrough which the mixed' gases flow vfrom saidl mixing lchambenvapor generating surface in the path ofthe mixed gasesowing from said chamben'meansf'orming a b'yj-pass in said vgas passage, means for' controlling the flow of gases through the by-passjand superheating surface insaid gaspassage. on both sides of said by-pass over vwhich the gases from'said mixing chamber f'low in passing through said passage,the arrangementbeing such that the superheat temperature is controlled by unequally firing the furnaces to regulater the temperature of the gas mixture and by regulating the quantity of gases'fiowing through said by-pass'.

` 5. Vapor generating apparatus comprising a setting, a plurality of furnaces in thel setting ary ranged "tocperatein parallel;` mea-ns for'I firing each furnace at various intensities, a water wall in eachfurnace, a mixing chamber intoy which the gases from eachffurnace fiow and mix with thegases from another furnace, means forming -a gas passage-through vwhich themixed rgases flow` from said mixing chamber, means yforming a -by-pass in said-gas passage, vapor generating surfacey in saidv byl-Dass, means for controlling the flow of gasesA throughthe by-pass, and superheating surface in the gaspassage on both sides of said by-pass over Vwhich the ygases from said mixing chamber flow in passing' through the passage, the arrangement being such 4that the superheat temperature is controlled by unequally firing the furnaces to regulate the temperature of .the gas mixture and by regulating thequantity of gases flowing through said by-pass.

6. Vapor generating" apparatus comprising a setting, a plurality vof'furnaces in the -setting arranged to operate in parallel, means for ring each furnace at various' intensities, a water wall in each furnace, a mixing chamber into which the gases from each furnace flow and mix with the gases from another furnace,I means forming'a-gas passage through which-the mixed gases fiow' from said mixing chamber, "means forming a by-pass in-said gas passage, vapor generating surface in the path' of .mixed gases vflowing from the chamber.. into they passage and by-pass, means for controllingthe owof gases through the by-pass, 'and superheating-'surface in the-gas passage on both sides of said by-pass overA which the gases fromthe mixing chamber flow in passing through the passage, the arrangement being suchgthat. the -superheat tem- .Pia'fu .1S `gSIQINSJQO.lled. by r.unequally ming .the

furnace to regulate the temperature of the ,gas

mixture and by regulating :the yquantity flowing through said by-.pass., v'7. Vapor generatingxapparatus comprising a setting, a plurality of furnaces in thesetting arranged to operateinparallel, means for firing each vfurnace at vVarious intensities, a water wall in each furnace, a mixing chamber yinto which the gases froml each furnace `fiow and mix with thegases from another furnace, means forming. ,a gas passage through which the mixed gases flow from said mixing. chamber, means of gases forming va by-pass for said passage andv disposed to divide the passage intok portions sep- .y

arated by the by-pass, vapor-superheating sur.- face in each portionof the gas passage in heat exchange'relationship with gases owing there-.y through from themixing chamber, and means for controlling theflow of .gases through the byx v8. Vapor generating apparatus comprising a ,l

setting, a plurality of furnaces in the setting arranged to operate in parallel, means for firing.y each furnace at various intensities, a'water wall in each furnace, a mixing chamber into which the gases .from each furnace ow and mix with thegasesV from another furnace, means forming a gas passage through which the mixed gases flow from said mixing chamber, means forming a by-pass for said passage and disposed to divide the passage into portions separated by the bypass, vapor lsuperheating surface in each portion of thefgas passage in heat exchange relationship with gasesflowing therethrough fromv the mixing chamber, andmeans. for controlling the flow of gases' through the'by-pass, the arrangementv beingr such; that the superheat temperature is controlled. by unequally firing the furnaces tofregulate the temperature of the gas mixture and by regulating thequantity of flowing through said bly-pass.y

9. Vapor generating apparatus comprising a setting, a plurality of furnaces in the setting arranged .tol operate in parallel, meansfor firing each furnace 'at'.various intensities, a Water wall in each furnace, a mixing `chamber intoY whichthe. gases from. each furnace flow and. mix with the gases from another furnace, means forming a gas passagethrough'which the mixed gases .flow from `thefrnixing chamber, means forming a by-pass for said passage and disposed to divide the passage into portions separated by the by-pass, Vapor superheating surface in each portion of the gaspassage in heat exchange relationship with gases flowing therethrough from the mixing chamber, means for controlling the now of gases through th'e by-pass, and a heat recovery` device. in at least one of -the portions of"v said passage in the path of the gases Vfiowing from `the superheating surface and from said the gas passage in heat exchangerelationship with gases flowing; therethrough from the mixing` gases 2,834,187 chamber, `a `Ifieai'rrecovery device in at least one` portion of the passage in the,y path of the gases ilowing from the superheating surface, the bypass having gas outlets positioned adjacent the heat recovery surface so that gases vdischarged from the by-pass will flow over the heat recovery device, and means associated with the ,gas outlets for controlling the flow, of gases through the by-pass. f l

11. Vapor generating` apparatus comprising a setting, a plurality of furnaces in the setting arranged to operate in parallel,V means for ring a bil-pass for said passage and disposed to divide the passage `into two substantially parallel portions separated by the by-pass, the by-pass having gas outlets at opposite sides thereof and in communication with the separate portions of said gas passage, vapor superheating surface in each portion of the gas passage in heat `exchange relationship with gases flowing therethrough from the mixing chamber, a heat recovery device in each of saidportions of the gas passage in the vpath of gases flowing from the superheating surface, baie means adjacent each gas outlet f of said by-pass adapted to direct the gases from the by-.pass over the heat recovery device, and means associated with the gas outlets for con-v trolling the flow of gases through the lluy-pass.

MAR'IIN FRIsCH. A 

